Pressure retaining valve

ABSTRACT

The invention relates to a pressure retaining valve including a housing which has a housing inlet passage and a housing outlet passage which can be connected to the housing inlet passage, a valve piston which is movable in the housing and has a valve shaft, a valve seat associated with the valve piston, whereby the valve piston is biased by a spring in the direction towards the valve seat and, in the closed position, seals the connection between the housing inlet passage and the housing outlet passage, and an isolating membrane which is clamped in the housing and separates the spring and the valve piston.

The invention relates to a pressure retaining valve including

a housing which has a housing inlet passage and a housing outlet passagewhich can be connected to the housing inlet passage,

a valve piston which is movable in the housing and has a valve shaft,

a valve seat associated with the valve piston, whereby the valve pistonis biased by a spring in the direction towards the valve seat and, inthe closed position, seals the connection between the housing inletpassage and the housing outlet passage, and an isolating membrane whichis clamped in the housing and separates the spring and the valve piston.

With pressure retaining valves of the type referred to above one refersto a flow beneath the valve seat which opens the valve piston. Inanother known type of construction, the housing inlet passage leads intothe interior of the housing between the isolating membrane and the uppersurface of the valve seat directed towards it, whereby the outletpassage commences at the underside of the valve seat; with this type ofconstruction one refers to a flow beneath the membrane which opens thevalve piston. In the case of flow beneath the membrane, so-calledcontrol oscillations do not occur as in the pressure retaining valves ofthe type referred to above but the flow beneath the membrane has themajor disadvantage that the entire membrane surface is necessarily actedupon by the entire prevailing system pressure so that enormous springforces must be applied by the valve spring closing the pressureretaining valve and enormous forces act on the valve seat or the valveseat seal in the event of a pressure reduction in the system down tozero and the isolating membrane constitutes a safety risk and must thusbe manufactured with a high manufacturing expense and strong textilereinforcements.

In the pressure retaining valves of the type referred to above, in whichflow occurs beneath the valve seat, the high forces do not occur; theisolating membrane needs only to protect the spring dome with itsindividual components from aggressive media. However, the occurrence ofcontrol oscillations in the pressure retaining valves of the typereferred to above is disadvantageous. In order to maintain the valvesealed, the spring force is adjusted so that it just overcomes the forcewhich is exerted by the medium on the closed valve piston and which isproduced by the product of the pressure of the medium and the exposedarea of the housing valve seat. In the event of a pressure rise beneaththe valve piston which forces the valve to open, a pressure will beproduced due to the internal resistance of the pressure retaining valveto the outlet side of the valve which can be small but which, when thevalve piston opens, acts beneath the membrane surface through aninternal space in the housing which, due to constructional andmanufacturing reasons, extends up from the housing outlet passage to theisolating membrane. The valve piston thus lifts up accordingly. However,since the spring force is sized or adjusted for the relatively smallexposed surface of the housing valve seat, a balance between the setspring force and the generated membrane force is not produced so thatthe valve piston is pressed again in the direction towards the housingvalve seat. This up and down movement of the valve piston occurs atrapid intervals; the pressure retaining valve rattles and causesundesired control oscillations.

Pressure retaining valves in which flow occurs beneath the valve seatare known in which complicated special valve piston constructions andvalve piston guides with automatically closing and opening bypasspassages are used in order to alleviate the problem of the controloscillations. The constructional and functional effort therefor is,however, high and expensive.

The invention therefore has the object of constructing a pressureretaining valve of the type referred to above with simple and economicalmeans so that no control oscillations occur.

The invention solves this object principally by the fact that

a guide disc which slidably guides the valve shaft is arranged betweenthe isolating membrane and the valve seat, and

that the guide disc surrounds the valve shaft whilst forming a narrowannular gap acting as a throttling point.

There is a connection between the upper side of the housing valve seatand the underside of the isolating membrane via the constructionallydetermined annular gap. When the valve piston opens, medium can flow inthis manner through the annular gap to beneath the isolating membrane.However, if a state of balance between the spring force and membraneforce is not reached and the valve piston wants to move in the directiontowards the housing valve seat, the medium must also be forced backthrough the narrow annular gap. The annular gap thereby gives rise to aneffective hydraulic damping so that the pressure retaining valveoperates surprisingly well and without oscillations, as practical testshave shown.

It has been found that an oscillation-free control performance, which isvery satisfactory in practice, may be achieved for most media used inpractice with pressure retaining valves if the diameter of the valveshaft is not substantially larger than the valve seat and the annulargap between the valve shaft and the guide disc has a free radial breadthwhich is at most 0.25 mm. The throttling action and freedom fromfriction are optimised in this manner.

An advantageous embodiment in accordance with the invention of thepressure retaining valve resides in that at its end directed towards thevalve seat the valve piston has a piston collar which is provided with adisc-shaped recess, that the recess has a U-shaped boundary edge andthat a sealing plate is inserted loosely in the recess with a smallclearance. As a result of the clearance the risk is avoided that apressure can build up behind the sealing disc which, in the event of apressure reduction in the pressure retaining valve in front of thesealing plate, could force the latter out of the recess. The sealingplate is thus mounted on the valve piston in a manner which is securedagainst washing out.

A preferred embodiment of the invention is characterised in that theeffective surface of the isolating membrane is substantially of the samesize as the surface of the valve seat. The surfaces should preferablydiffer from one another by at most eight percent. This embodiment hasthe substantial advantage that the pressure retaining valve is resistantto back pressure. If a back pressure occurs in the housing outletpassage, a first force directed in opposition to the spring force actson the membrane, the magnitude of which is equal to the product of theeffectove area of the isolating membrane and the back pressure.Furthermore, the back pressure exerts on the surface, which is directedtowards the isolating membrane, of the valve piston of the size of thevalve seat a second force whose magnitude is equal to the first force,due to the same size of the effective area of the isolating membrane andthe area of the valve seat, and is directed in opposition to it. Thesetwo forces cancel one another out so that even in the event of a backpressure no additional force is exerted on the spring, as would be thecase if the effective area of the membrane were larger than the area ofthe valve seat. The inlet pressure in the pressure retaining valve canbe maintained constant in this manner independently of the prevailingback pressure. Further advantageous embodiments are characterised in thedependent claims.

The invention will be explained in more detail below by way of twoexemplary embodiments shown in the drawings, in which:

FIG. 1 is a schematic axial sectional view of a first exemplaryembodiment of the pressure retaining valve in accordance with theinvention and

FIG. 2 is a similar view of a second exemplary embodiment of thepressure retaining valve in accordance with the invention.

As shown in FIG. 1, a valve housing 1 includes a valve housing space 2with a housing valve seat 3. Leading to the underside of the valve seat3 is a housing inlet passage 4. Commencing at the upper side of thevalve seat 3 is a housing outlet passage 5. The pressure retaining valvefurther has a spring dome 6 with a compression spring 7 which presses avalve piston 8 against the valve seat 3 with its adjustable springforce. Sealingly clamped between the housing 1 and the spring dome 6 isan isolating membrane 9 which separates the valve housing space 2 andthe valve piston 8 from the spring dome 6 and the compression spring 7.

Arranged in the valve housing space 2 above the valve seat 3 is a guidedisc 10 which slidably guides the valve piston 8 and surrounds itscylindrical valve shaft 11 such that it defines a narrow annular gap 12together with the guide shaft extending over the entire periphery of theshaft. The guide disc 10 covers the underside of the isolating membrane9, with the exception of this narrow annular gap 12, in a sealed mannerwith respect to the upper surface of the valve seat 3 connected to thehousing outlet passage 5 so that medium can only flow to the undersideof the isolating membrane 9 through the narrow hydraulic clearance ofthe annular gap 12. The radial breadth of the annular gap 12 is so smallthat the medium can only flow through the annular gap 12 with a flowwhich is strongly decelerated and effectively damped by throttling,depending on its viscosity. The desired oscillation-free controlperformance without rattling of the pressure retaining valve is achievedby this simple constructional feature alone. This object can be achievedin most cases occurring in practice in which pressure retaining valvesare used if the free radial breadth of the annular gap 12 between thevalve shaft 11 and the guide disc 10 is at most 0.25 mm, regardless ofwhat size the valve piston has in dependence on the nominal breadth ofthe pressure retaining valve.

The valve shaft 11 has a diameter which substantially corresponds to thediameter of the valve seat opening 3. At its end directed towards thevalve seat 3 the valve shaft 11 has a piston collar 14 whose diameter isgreater than the valve seat opening 3. The piston collar 14 includes adisc-shaped recess 15 in which a sealing plate 13, which covers thevalve seat opening 3, is loosely inserted. The boundary edge of therecess 15 is of approximately U-shape and embraces the peripheral edgeof the sealing plate 13 not in a clearance-free and sealed manner butwith a small movability clearance. As a result of this clearance mediumwhich has flowed behind the sealing plate can escape again unimpeded tothe front side of the sealing plate. A mounting of the sealing plate 13in the disc-shaped recess 15 which is secure against washing out is thusachieved in this manner.

The pressure retaining valve in accordance with FIG. 2 differs from thepressure retaining valve in accordance with FIG. 1 only in that theisolating membrane 9' has a smaller effective area than the isolatingmembrane 9 in FIG. 1. The isolating membrane 9' is so clamped that itseffective area is of the same size as the area of the valve seat 3. Acounter pressure possibly present in the housing outlet passage 5 doesnot impair the function of the pressure retaining valve in this manner.The force exerted on the isolating membrane by the back pressure, themagnitude of which is equal to the product of the effective area of theisolating membrane and the back pressure, is thus equal to theoppositely directed force exerted on the valve piston. The valve is backpressure resistant in this embodiment and thus operates with very highaccuracy.

Numerous variations are possible within the scope of the inventiveconcept. The valve shaft and the associated annular gap can e.g. haveany desired diameter depending on the size of the valve.

I claim:
 1. Pressure retaining valve comprising:a housing which has ahousing inlet passage, a housing outlet passage and a connecting passagein between, a valve piston which is movable in the housing and has avalve shaft, a valve seat being associated with the valve piston, thevalve piston being biased by a spring in the direction towards the valveseat and, in the closed position, sealing the connecting passage betweenthe housing inlet passage and the housing outlet passage, an isolatingmembrane which is clamped in the housing and separates the spring andthe valve piston, a guide disc arranged between the isolating membraneand the valve seat, the guide disc surrounding and slidably guiding thevalve shaft, and the guide disc and the valve shaft forming a narrowannular gap acting as a throttling point.
 2. Pressure retaining valve asclaimed in claim 1, wherein the diameter of the valve shaft is notsubstantially larger than the valve seat andthe breadth of the annulargap between the valve shaft and guide disc is at most 0.25 mm. 3.Pressure retaining valve as claimed in claim 1, wherein at its enddirected towards the valve seat the valve piston has a piston collarwhich is provided with a disc-shaped recess,the recess having a U-shapedboundary edge and a sealing plate loosely inserted with a smallclearance into the recess.
 4. Pressure retaining valve as claimed inclaim 1, wherein the housing comprises two portions,one of the housingportions being associated with the spring and constructed as a springdome and the isolating membrane being clamped between the two housingportions.
 5. Pressure retaining valve as claimed in claim 1, wherein theisolating membrane has an effective area of substantially the same sizeas the area of the valve seat.